There are many times both during and after the construction of long tubular members when it is desirable to examine the member, for example, to determine whether there are any structural faults, and if a fault does exist, to determine its location. In most applications, it is not difficult either to visually examine the member from its exterior or to pass an examining device, for example, an X-ray apparatus, along the exterior of the member to determine whether the member is in a satisfactory operating condition.
It is difficult, if not impossible, however, to examine the member from its exterior when the member is either buried beneath the surface of the earth or buried deep beneath the surface of a body of water. A typical application is examination of a pipeline laid along or buried beneath the bottom of a body of water, such as those used to bring offshore petroleum or gas production onshore.
Thus, in the process of laying and maintaining these underground or underwater pipelines, it is often necessary, as a routine maintenance procedure, to examine tubular members by passing a pig member through the pipeline interior, the exterior of the member being substantially inaccessible. In other situations, the pig member traverses the interior of the pipeline to detect a known buckle that has occurred a substantial distance from the pipeline entrance opening. A typical buckle detector for this application is described and claimed in Jones, U.S. Pat. No. 3,739,591, issued June 19, 1973, and assigned to the assignee of this invention. The pig member described in Jones is driven through the pipeline interior by a pneumatically actuated drive system.
During traversal of the pipeline, however, the pig member, such as that described in Jones, U.S. Pat. No. 3,739,591, tends to rotate around its longitudinal axis. Consequently, when two separate lines are connected to the pig described in Jones, U.S. Pat. No. 3,739,591, for example a tow line and a fluid flow line, the lines rotate about one another and tend to twist and entangle because of the rotation of the pig as it travels. The entangled lines may either cause a loss of fluid pressure at the pig member thereby stopping its forward progress, or may prevent the proper detachment of one of the lines which normally occurs after the pig member reaches its termination portion within the tubular member.
It is therefore a principal object of this invention to provide an apparatus and method for reducing and even preventing interline twisting when a pig member traverses the interior of a tubular member. Other objects of the invention include providing a reliable and low cost apparatus and method for preventing interline twisting, providing an apparatus and method which can be used under water as well as underground, and which will operate at high pressures. Yet further objects of the invention are to provide an apparatus and method which maintains substantially twist-free line pairs and has a simple structure.